class_docstr: azure.storage.blob.BlobServiceClient objects internally cache HTTPS connections.
  In order to reuse HTTPS connections, we need to reuse BlobServiceClient objects.
  
  The effect we are going for is for each process or thread in the executor to
  EACH REUSE ITS OWN long-lived BlobServiceClient object.
code: def __init__(self):
      raise RuntimeError("_ClientCache may not be instantiated!")
  
file_path: metaflow/metaflow/plugins/azure/blob_service_client_factory.py
imports: from metaflow.exception import MetaflowException
  from metaflow.metaflow_config import AZURE_STORAGE_BLOB_SERVICE_ENDPOINT
  from metaflow.plugins.azure.azure_utils import create_cacheable_default_azure_credentials, check_azure_deps
  import os
  import threading
label_desc: Use image node_id 1 to create a new _ClientCache object from inherited base classes: object with example: obj = _ClientCache()